Fellow: Ursinio Puga
Adviser: Wei Lin

The City of Fargo owns and operates the only wastewater treatment plant (WWTP) in Fargo, North Dakota, United States. The WWTP was built in 1934 and it has been expanded several times since its construction. Currently, the WWTP treats an average daily flow of 15 million gallons per day (MGD) and has the ability to handle a peak pumping capacity of 29 MGD. The Fargo WWTP will yet undergo another expansion, increasing its peak pumping capacity from 29 to 50 MGD.

The current method used to disinfect the wastewater at the Fargo WWTP is chlorination followed by de-chlorination. The expansion requires the construction of an additional chlorine contact basin to properly treat the future flow. Due to the close proximity to the Red River, there is extremely limited space for a new contact basin on the existing WWTP’s campus. After studying several alternatives, the engineers designing the plant’s expansion recommended switching to a UV disinfection system. According to the consulting firm, retrofitting the existing disinfection contact basin into a UV system will allow the plant to provide adequate disinfection to its wastewater without having to increase the footprint of the existing disinfection contact basin to handle the future flow.

Although UV disinfection is a proven technology, its application in wastewater treatment is relatively new in North Dakota and throughout the Midwest. Additionally, past research indicates that there is a need to develop on-site studies to effectively design a full-scale UV disinfection system for a particular plant depending on its flow and water quality characteristics. The management of the Fargo WWTP decided to perform on-site studies to determine the UV disinfection efficiency under different flow conditions, potential seasonal water quality changes, and quartz sleeve fouling prior to start implementing this technology in a full-scale basis.

It was decided that the best way to perform on-site studies was through a pilot study. Fargo’s WWTP management proposed a 7-month pilot study (April 1st through October 31st 2016) to match the current Fargo’s WWTP disinfection season. The pilot was carried out by North Dakota State University researchers with collaboration with the scientists and wastewater experts from the City of Fargo, engineers from Apex Engineering Group, and representatives from UV system manufacturers. The specific objectives of the study are as follows:

1. To evaluate the impact of effluent water quality change on UV Transmittance (UVT);
2. To evaluate the impact of UVT changes on UV disinfection of E. coli;
3. To evaluate the fouling tendency of quartz sleeves; and 38
4. To evaluate the impacts of fouling and flow rate variations on UV disinfection of E. coli.

A pilot unit was used to determine the UV disinfection efficiency under different flow conditions, potential seasonal water quality changes, and UV lamp fouling material deposition. Additionally, a collimated beam (CB) apparatus was used to find the relationship between UV dose and E. coli inactivation efficiency in wastewater for different UVTs.

Significance of Research

Previous studies show that absorption of UV light by dissolved substances is a major contributor to the reduction UV light transmission in wastewaters. Dissolved substances interfering with the operation of the UV disinfection system was a major concern for the Fargo WWTP management since the city applies ferrous salts in its sewer system for odor control. Both the WWTP management and the engineering consulting firm anticipated that some of the iron added throughout the collection system may make their way through the plant and end up in the effluent in dissolved or particulate form hindering UV disinfection performance through absorption of UV light. The motivation that lead the WWTP management to proposing this pilot study was to investigate the behavior of the fouling material accumulation on the surface of the quartz sleeves and whether it was impacted by the iron added to the system. On the other hand, engineers from Apex Engineering Group supported the idea of performing a pilot study to obtain parameters needed to improve the design of the future full-scale UV disinfection system that will be installed on-site. All the data collected during this research effort will be made available to the City of Fargo as well as to the Apex Engineering Group as a basis to develop operation strategies and to improve the design of the future full-scale UV disinfection system.

Significant Findings

Instead of using the commonly accepted first-order kinetic model, a second-order kinetic model was successfully applied to explain the experimental results obtained with the CB apparatus. By using the second-order model we were able to quantify the impact of UVT on E. coli inactivation and to properly explain the impact of initial E. coli concentration on its inactivation rates. Reduction of UV intensity due to fouling of the quartz sleeves was a major concern during the UV system operation. Results of the pilot system showed that fouling was caused mostly by precipitation of metal salts and the impact of fouling on UV intensity reduction was successfully explained by the application of the Beer-Lambert law. E. coli inactivation in the pilot reactor was found to be dependent on influent UVT, flow rate, and UV intensity. The first-order plug flow model was used to interpret the pilot study results. No significant seasonal water quality changes that may affect the UV system operation were identified. However, water quality changes due to storm events could cause short term adverse impact on the UV system performance.